This invention relates generally to underwater sound generators, and more particularly to generators of low-frequency sound waves utilizing the fluid properties of ferrofluids.
Many years of research have been spent in a search for more efficient sonar equipment, but basically transducers for converting electrical energy into acoustic radiation in a fluid medium have been limited to magnetostrictive, piezoelectric, and moving-coil types. These sonar transducers have many shortcomings with respect to the production of low-frequency sound.
Within the size, weight, and mechanical constraints of the sonar application, any transducer intended for low-frequency sound generation will be small when compared with the acoustic wavelength in the medium. This condition implies a very low radiation resistance. The power radiated from a transducer whose dimensions are small with respect to the wavelength of the sound generated is determined by the radiation resistance acting on, and the volume velocity generated by, the radiating surface. Since the radiation resistance decreases with decreasing frequency, low-frequency acoustic sources must have a large volume velocity. Thus, the final compromise left to the transducer designer is a choice between increasing the transducer surface velocity or its radiating surface area. All magnetostrictive, piezoelectric, and moving-coil transducers suffer from the fact that they are surface velocity (or displacement)-limited. Therefore, to maintain a given power output, the surface area of the transducer must increase rapidly with decreasing frequency. However, if the surface area of the transducer is increased, the forces due to hydrostatic pressure will also increase, and a heavier device will result from the reuired pressure-compensation mechanism or required increase in the volume of structural materials. Another disadvantage is that the requirement for large volume velocities means that low-frequency sources are inherently large force devices and, as such, when utilizing magnetostrictive, piezoelectric, and moving-coil mechanisms, or other means of mechanical transformation, are generally not reliable over their operational lifetime.
In addition, the production of low-frequency sound is limited in prior-art devices in the following ways. Piezoelectric devices are brittle and break easily from shock or excessive displacements. Magnetostrictive materials have low displacement capabilities and efficiency except for certain rare earth-iron alloys which are extremely expensive and are not available in large quantities at any cost. Magnetostrictive and piezoelectric devices contain materials that are extremely heavy, even when submerged in water. Moving-coil devices require extensive pressure-compensation systems that limit operational depth and depth-cycling. Other mechanical devices such as hydraulic rams are very complicated and expensive. Transducers using controlled explosions are frequency-limited and non-reproducible.